Defense in Monster Hunter World (MHW) is a cumulative stat that equals the total protection the player or monster has against Raw Damage. Raw Damage is also known as Attack Power and referencing that page will assist you in understanding the calculations involved in determining damage taken.

Improve Defense in Monster Hunter World

Players can equip Armor that increases their defense against Attack Power

Players can equip Charms that increase their defenses against Attack Power

Players can seek out Skills that increase their defense against Attack Power

Little side note, elemental resistance might play a role in how much damage you take defense wise, after reducing my defense in favor of elemental resistance, I took pretty much the same amount of damage, so defense appears to be increased meekly and affected in some way by elemental resistance, so defense is important but is weakened if you don't have enough elemental resistance, it might not even play a role unless it's at 10 or 20 to boot, but that's just me and my experiments, anybody else have some other crazy idea about defense and elemental resistance?